In recent years, composite materials comprising highly filled polymers have become commonly used for dental restorations. Current composite materials contain crosslinking acrylates or methacrylates, inorganic fillers such as glass or quartz, and a photoinitiator system suitable for curing by visible light. Typical methacrylate materials include 2,2′-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (“Bis-GMA”); ethoxylated Bisphenol A dimethacrylate (“EBPDMA”); 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (“UDMA”); dodecanediol dimethacrylate (“D3MA”); and triethyleneglycol dimethacrylate (“TEGDMA”). The structural formulae for these are shown below.

Dental composite materials offer a distinct cosmetic advantage over traditional metal amalgam. However, they do not offer the longevity of amalgam in dental fillings. The primary reasons for failure are excessive shrinkage during photopolymerization in the tooth cavity, which causes leakage and bacterial reentry. Another reason is they have inadequate strength and toughness, as reflected in the measured properties of flexural strength and fracture toughness. Hence, there is still a need for new monomers and new monomer combinations which, when polymerized, impart high fracture toughness and flexural strength in the resulting composite. It is also highly desirable to have low shrinkage and low shrinkage stress on polymerization.
One of the more common commercially used monomer is Bis-GMA, making it an especially important monomer in dental composites. However, it is highly viscous at room temperature and is insufficiently converted to polymer when cured. It is therefore diluted with a second, lower viscosity polymerizable component, typically an acrylate or methacrylate monomer, such as trimethylol propyl trimethacrylate, 1,6-hexanediol dimethacrylate, 1,3-butanediol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, TEGDMA, or tetraethylene glycol dimethacrylate. However, while providing low viscosity, lower viscosity components (generally low molecular weight monomers) contribute to increased shrinkage. Increasingly, Bis-GMA and TEGDMA have been combined with UDMA and EBPDMA, but shrinkage remains high enough that improvement is desirable.
In the search for superior dental composites, many research groups have looked to new monomers. For example, Culbertson describes the synthesis of trimethacrylate dental monomers derived from 1,1,1-tris(4-hydroxyphenyl)ethane (THPE). Culbertson treats THPE with ethylene or propylene carbonate, then caps the hydroxyl group with methacrylic anhydride:
The resulting compounds, 1,1,1-tri[4-methacryloxyethoxy)-phenyl]ethane (“THPE EO MA”) when R═H and 1,1,1-tri[4-2-methyl-2-methacryloxyethoxy)-phenyl]ethane (“THPE PO MA”) when R=methyl, are tested in dental composites. A 70/30 THPE PO MA/TEGDMA composite (TM7T3) has a shrinkage of 2.48%, while a 70/30 Bis-GMA/TEGDMA composite (Control 2) has a shrinkage of 3.28%. However, the flexural strength (113 MPa) is not improved over Control 2 (112.7 MPa). See J. Macromol. Sci. Pure Appl. Chem. (2002), A39(4), 251-265.
Chung et al. describe the synthesis and polymerization of trifunctional 20 methacrylates derived from 1,1,1-tris(4-hydroxyphenyl)ethane triglycidyl ether (“THPE GE”) and their application as dental monomers. They are formed by treating THPE GE with methacrylic acid and then optionally acetylating the hydroxyl group. A disadvantage of these monomers is their high viscosity as compared with that of Bis-GMA. For example the product below (R═H) has a viscosity of 3510 Pa.s at 25° C. The acetylated compound (R=Ac) has a viscosity of 2810 Pa.s at 25° C. In comparison to Bis-GMA, whose viscosity is 54.7 Pa.s at 25° C., these monomers are much more viscous, which may limit their use in some composite formulations.
See J. Biomed. Mater. Res. (2002), 62(4), 622-627 and Biomaterials (2003), 24(1), 3845-3851.
Branched polyester methacrylates are another class of new dental monomers. For example, Culbertson, et al. used a variety of synthetic routes to methacrylate Boltorn H30, a commercially available polyester polyol with a dendritic structure (Perstorp AB, Perstorp, Sweden) that is synthesized by a condensation reaction of a pentaerythritol core with 2,2-dimethylolpropionic acid. The methacrylated Boltorn H30 was intended as a replacement for at least some of the Bis-GMA in dental composite materials. Culbertson, et al. evaluated the resulting partially and fully methacrylated materials as dental composite material components by mixing them in varying proportions with a 50:50 mixture of Bis-GMA and TEGDMA or with TEGDMA without Bis-GMA, and photopolymerizing the mixture. Resins made from a 50:50 mixture of methacrylated Boltorn H30 and TEGDMA had lower linear polymerization shrinkage than the 50:50 Bis-GMA/TEGDMA control. However, compressive strength and flexural strength were typically lower than the control. Since no filler was present, it is difficult to use these results to predict how such materials would perform in actual dental composite materials. See Culbertson et al., J. Macromol. Sci. Pure Appl. Chem. (2000), A37(11), 1301-1315.
Another class of materials is macromonomers (see definition below) with olefinic end groups. These are described by, for example, Macromolecules (1996), 29, 7717. These materials are usually prepared by polymerization of methacrylate monomers in the presence of a “catalytic chain transfer” (CCT) catalyst. The catalyst is typically a chelated cobalt species. Macromonomers have been described for use in automotive coatings, but not for dental composite applications.
There remains a need for dental composite materials that combine reduced shrinkage with sufficiently low viscosity, high polymerization rate, and acceptable mechanical properties.